An information-theoretic definition of similarity proposed by Dekang Lin in 1998 appears to have attractive properties for identifying medically similar cases in a large database. Examination of similar cases that appear following vaccination may provide a way of detecting previously unexpected associations. A key element here is the inclusion of shared time-since-vaccination as a dimension of similarity while considering concordance on many other features. The system is potentially much more sensitive to patterns with multiple features than are very large, high-dimension contingency tables. Copyright © 2009 John Wiley & Sons, Ltd.